1. Field of the Invention
The present invention relates to a semiconductor memory device. More particularly, the present invention relates to a nonvolatile semiconductor memory device having a gate with a different film structure from a silicon-oxide/nitride/oxide-silicon (SONOS), and a method of manufacturing the same.
2. Description of Related Art
Recent developments of portable data storage devices, such as memory sticks, that readily allow data sharing and exchange regardless of hardware type have led to increased demands regarding safe storage of large amounts of data. One possible solution is a flash memory device, i.e., a special type of an electrically erasable programmable read-only memory (EEPROM), which can be erased or programmed in blocks of data and the contents of which are retained even when the power is turned off. The flash memory device generally has a multi-layer structure including a floating gate, where a charge is stored, a transistor gate, where data are stored, and a control gate that controls the floating gate, the gates being sequentially deposited. However, the flash memory has a low retention characteristic and data stored therein cannot be kept safely for a long period of time due to a leakage current.
To solve this problem, a silicon-oxide/nitride/oxide-silicon (SONOS) memory device having a reduced height has been introduced. The SONOS memory device uses stacked layers between a substrate and a control gate. That is, instead of the floating gate positioned between insulating layers in the flash memory, the SONOS memory device uses a stacked layer made by sequentially depositing an oxide film, a nitride film, and another oxide film (ONO). The SONOS memory device operates by shifting a threshold voltage when a charge is trapped in the nitride film. A detailed description of a SONOS memory device is disclosed in an article entitled “An Embedded 90 nm SONOS Nonvolatile Memory Utilizing Hot Electron Programming and Uniform Tunnel Erase,” by C. T. Swift et al., which was published in the Technical Digest of International Electron Device Meeting (IEDM) December 2002, pp. 927-930.
FIG. 1 illustrates a cross-section of a basic structure of a conventional SONOS memory device.
Referring to FIG. 1, a first silicon oxide (SiO2) film 12 is formed on a channel region between a source region S and drain region D of a substrate 10. One end of the first silicon oxide film 12 contacts the source region S and another end of the first silicon oxide film 12 contacts the drain region D. The first silicon oxide film 12 is for tunneling a charge. A nitride (Si3N4) film 14 is formed on the first silicon oxide film 12. The nitride film 14, which is practically a material film for storing data, traps the charge tunneled through the first silicon oxide film 12. A second silicon oxide film 16 is formed as a barrier film for blocking the movement of the charge from the nitride film 14 upward through the nitride film 14. A gate electrode 18 is formed on the second silicon oxide film 16.
Although the conventional SONOS device depicted in FIG. 1 provides some advantages, it also has the following drawbacks.
In practical use, an operating voltage of the conventional SONOS device is very high. If a low operating voltage is applied to the conventional SONOS device, the speed of writing/erasing of data becomes much slower than a desired speed. Due to such voltage dependence characteristic, the control of a trap density of the nitride film 14 also becomes difficult. The retention time also is not sufficiently long.
These drawbacks arise due to the high thickness of the films required by the use of nitride and silicon oxide, which have low dielectric constants.
Recently, a paper by C. Lee et al., entitled “Novel Structure of SiO2/SiN/High-k dielectric, Al2O3 for a SONOS Type Flash Memory,” published in Extended Abstract of 2002 International Conf. on Solid State Device and Materials, Nagoya, Japan, September 2002, pp. 162-163, reports that programming, erasing, and retention characteristic of the barrier film can be improved by using an aluminum oxide film (Al2O3) instead of an silicon oxide film as the upper oxide film. However, the applied voltage is still very high and it is still difficult to control the trap density of the silicon nitride film.